Encrypting Machine

Encrypting Machine

Ellie built an encrypting machine. In was rudimentary, but it worked. It could take and English-language message and transform it by a simple substitution cipher into something that looked like gibberish. Building a machine that would do the reverse – converting an encrypted message into clear when you didn’t know the substitution convention – that was much harder.

You could have the machine run through all the possible substitutions (A stands for B, A stands for C, A stands for D…), or you could remember that some letters in English were used more often than others. You could get some idea of the frequency of letters by looking at the sizes of the bins for each letter of type in the print shop next door.

The war still has to be fought even though we are behind barbed wire. (Quotations from M. Jorgensen). Image: © Megan Jorgensen (Elena)

ETAOIN SHRDLU, the boys in print shop would say, giving pretty closely the order of the twelve most frequently used letters in English. In decoding a long message, the letter that was most common probably stood for an E. The most common three-letter word in the language was “the”. If within a word there was a letter standing between a T and an E, it was almost certainly H. If not, you could bet on R or a vowel.

Ellie deduced other rules and spent long hours counting up the frequency of letters in various schoolbooks before she discovered that such frequency tables had already been compiled and published. Her decrypting machine was only for her own enjoyment. She did not use it to convey secret messages to friends. She was unsure to whom she might safely confide these electronic and cryptographic interests, the boys became jittery or boisterous, and the girls looked at her strangely.